The ADIRU supplies primary flight data, inertial reference, and air data. The ADIRU is fault–tolerant and fully redundant.
The SAARU is a secondary source of critical flight data for displays, flight control systems, and other systems. If the ADIRU fails, the SAARU automatically supplies attitude, heading, and air data. SAARU heading must be manually set to the standby compass magnetic heading periodically.
The SAARU also supplied data for the standby instrument.
Most autopilot/FD modes become unavailable is the IRS part of the ADIRU fails, as the SAARU cannot supply the required data, including LNAV, VNAV, TOGA, LOC and GS (!) - you need to fly the ILS raw data! - TRK and HDG.
So, with a complete ADIRU failure, you are looking at flying mostly manually.